Tracing water leaks is one of the most challenging things that one might have to do with a structure.
Water can travel quite a distance and in completely non-intuitive paths. It is especially difficult to trace where two materials are butted against each other, like a pair of 2x4s side by side. In this situation, water can travel laterally or upward through capillary action (also known as wicking), as well as down.
How to do it varies considerably depending on circumstances. However, you might try a bright light shone where you see water and try closely watching for movement. Sometimes wiping a small part of it is needed to introduce a break so you can see its movement. Follow it upstream as far as you can. When (not if) you lose the trail, make a best guess where it is coming from and try to reacquire it there. Repeat.
Being successful may require a lot of patience. In one place I know of, the management company spent over $20,000 for experts to trace a slow drip into the middle of a condominium ceiling on the first floor. It turned out to be a window on the second floor about 25 feet away. The investigation involved tearing up a lot of decking, ceiling, two doorways, and a lot of closets.
Tyler's suggestion is a good one, but not very convenient nor comfortable during stormy weather. But you might try the inverse: Obtain a tarp (saran wrap, painting drop cloth, etc.) and cover the window exterior and see if that stops the water. If it does, maybe you can cover progressively less than the whole window to isolate the source.